[Documentation] Clarify Query Support location and `transform_flow`

[badmonster0]

Posted from discord by prashanth, it is a great suggestion

It would make sense to place this section in between the "CocoIndex core" and "Built-in sources"? That would make it easier to find.

[georgeh0]

No strong opinion. Personally I think the location is too high. Also previously we put it near HTTP server as it's only useful when using CocoInsight. (not sure if it's a good idea to create another "CocoInsight" folder?)

[badmonster0]

additionally

The location of the transform_flow decorator is deeply buried inside the "Query support" section.

Search bar in the docusaurus site doesn't find it, so it's quite hard to know why this decorator exists and when it should be used. I think it makes sense to include this decorator in the "Flow definition" section of the core docs, too, to indicate that this decorator is useful for both indexing and querying